Silicon carbide whisker reinforced ceramic composites and method for making same

ABSTRACT

The present invention is directed to the fabrication of ceramic composites which possess improved mechanical properties especially increased fracture toughness. In the formation of these ceramic composites, the single crystal SiC whiskers are mixed with fine ceramic powders of a ceramic material such as Al 2  O 3 , mullite, or B 4  C. The mixtures which contain a homogeneous disperson of the SiC whiskers are hot pressed at pressures in a range of about 28 to 70 MPa and temperatures in the range of about 1600° to 1950° C. with pressing times varying from about 0.75 to 2.5 hours. The resulting ceramic composites show an increase in fracture toughness of up to about 9 MP.am 1/2  which represents as much as a two-fold increase over that of the matrix material.

This invention was made as a result of work under Contract No.W-7405-ENG-26 between the U.S. Department of Energy and Union CarbideCorporation, Nuclear Division.

BACKGROUND OF THE INVENTION

Generally, the present invention relates to ceramic composites and thepreparation thereof, and more particularly to such composites in whichsingle crystal silicon carbide whiskers are dispersed to providedimprovements in the fracture toughness and fracture strength of theceramic.

Recent emphasis has been placed upon the use of ceramic materials asstructural components in heat engines and high-temperature conversionsystems such as turbines. For the use of ceramic in such applicationsfracture toughness (K_(Ic)) of the material is a critical consideration.Conventional ceramic materials have relatively low fracture toughnesswith the exception of Al₂ O₃ --ZrO₂ and partially stabilized ZrO₂.Monolithic ceramic material such as SiC, Si₃ N₄, Al₂ O₃ and mullite(3Al₂ O₃.2SiO₂) have a fracture toughness in the order of about 3 to 5MPa.m^(1/2) and a fracture strength (σ_(f)) in the range of about 30-100ksi (210-700 MPa). Utilization of these ceramic materials for thefabrication of structural components for use in heat engines and otherhigh-temperature conversion systems required the use of ceramiccomponents with very small flaw size less than about 50 μm) in order toprovide acceptable fracture toughness. However, in structural componentsespecially of complex configuration, the determination of such smallflaw sizes has been very difficult to achieve by using nondestructiveinspection techniques.

Efforts to overcome the lack of sufficient fracture toughness in ceramicmaterial included the development of fiber-reinforced composites. Forexample, graphite fiber reinforced ceramics provided impressive fracturetoughness and strength at ambient temperatures but these ceramiccomposites were found to be of questionable value when subjected toelevated temperatures because of the oxidation of the carbon fibers andthe reaction between the carbon in the fibers and the constituents ofthe ceramic material. On the other hand, the use of inorganic fiberssuch as silicon carbide (SiC) filaments and chopped fibers forreinforcing or strengthening ceramic material exhibited some success butencountered several problems which considerably detracted from theiruse. For example, conventional silicon carbide filaments or choppedfibers are of a continuous polycrystalline structure and sufferconsiderable degradation due to grain growth at temperatures about about1250° C. which severely limited their use in high temperaturefabrication processes such as hot-pressing for producing ceramiccomposites of nearly theoretical density. Further, during high pressureloadings such as encountered during hot pressing, the polycrystallinefibers undergo fragmentation which detracts from the reinforcingproperties of the fibers in the ceramic composite. Also, thesepolycrystalline fibers provided insufficient resistance to cracking ofthe ceramic composite since the fibers extending across the crack lineor fracture plane possess insufficient tensile strength to inhibit crackgrowth through the composite especially after the composite has beenfabricated by being exposed to elevated pressures and temperatures as inhot pressing.

SUMMARY OF THE INVENTION

Accordingly, it is the primary aim or objective of the present inventionto provide ceramic composites strengthened with silicon carbide whiskerswhich provide the ceramic composites with improved and more predictabletoughness characteristics than heretofore obtainable. Generally, theceramic composites which are characterized by increased toughness andresistance to fracture or cracking comprises a matrix of ceramicmaterial having homogeneously dispersed therein about 5-60 vol. % ofsilicon carbide whiskers These whiskers have a monocrystalline or singlecrystal structure and are in a size range of about 0.6 micrometers indiameter and a length of about 10-80 micrometers.

The ceramic composites are prepared by hot pressing a homogeneousmixture of particulate ceramic material and the silicon carbide whiskersat an adequate pressure and temperature to provide the composite withthe density of greater than 99% of the theoretical density of theceramic material.

The use of the single crystal whiskers in the ceramic composite providea significant improvement in the fracture toughness of the composite dueto their ability to absorb cracking energy. More specifically, in aceramic matrix where the SiC whisker-matrix interface sheer strength isrelatively low as provided by radial tensile stresses across thewhisker-matrix bond a process termed "whisker pull-out" occurs duringcracking to absorb the cracking energy and effectively reduce thetendency to crack and also inhibit crack propagation. Whisker pull-outoccurs as the matrix is subjected to crack-forming stresses. As thecrack-front propagates into the composite many of the whiskers whichspan the crack line and extend into the ceramic matrix on opposite sidesof the crack must be either fractured or pulled out of the matrix inorder for the crack to grow or propagate through the ceramic. Since thesingle crystal SiC whiskers possess sufficient tensile strength so as toresist fracturing they must be pulled out of the matrix for the crack topropagate. As these whiskers are pulled out of the matrix they exhibitconsiderably bridging forces on the face of the crack and effectivelyreduce the stress intensity at the crack tip so as to absorb thecracking energy.

The use of the SiC whiskers in ceramic matrices as will be described indetail below provide fracture toughness of the ceramic composites in arange of about 4-9 MPa.m^(1/2) and fracture strength in the range ofabout 38-120 ksi depending on the ceramic matrix material containing theSiC whiskers. This represents a considerable increase in fracturetoughness which in some instances is a factor of 2 or greater over thefracture toughness of the monolithic ceramic material used in thecomposite which is in the range of about 2-4 MPa.m^(1/2).

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative embodiments and methods about to bedescribed or will be indicated in the appended claims, and variousadvantages not referred to herein will occur to one skilled in the artupon employment of the invention in practice.

DETAILED DESCRIPTION OF THE INVENTION

As generally described above, the present invention is directed toceramic composites which exhibit improved mechanical properties,particularly increased fracture toughness, and to the manufacture ofsuch composites.

In accordance with the present invention ceramic composites are preparedby forming a mixture of ceramic powder or particulates withmonocrystalline SiC whiskers uniformly dispersed in the mixture. Themixture is then hot pressed to form a ceramic-SiC whisker composite of atheoretical density essentially equal to the theoretical density of thematerial.

The ceramic material found to be particularly useful for fabricating theceramic composites of the present invention which exhibit increasedtoughness over the monolithic form of the ceramic material include Al₂O₃, mullite (3Al₂ O₃.2SiO₂) and B₄ C. While it is expected that otherceramic material such as Si₃ N₄ or cordierite (2MgO.2Al₂ O₃.5SiO₃) maybe used to provide ceramic-SiC whisker composites of increased toughnesspreliminary results indicate that ceramic-SiC whisker composites formedwith ceramic materials such as Al₂ O₃.ZrO₂, ZrO₂, Si₃ N₄ (with Al₂ O₃and Y₂ O₃ dopants), and SiC (doped with B and C) do not exhibitsignificant toughening even though the composite densities greater thanabout 94% of the theoretical density were provided by hot-pressingcomposites of these ceramic materials.

The mixing of the SiC whiskers with ceramic powders to provide ahomogeneous dispersion of the whiskers in the matrix powders wasdifficult to achieve due to the tendency of forming undesirableagglomeration or whisker clumping during the mixing step. It has beenfound that ceramic matrix powders in a size range of about 0.5 to 1.0micrometers are preferred to form the homogeneous mixture since thetendency of whiskers to agglomerate is less than these fine powders.However, with improved mixing techniques it is expected thatagglomerate-free homogeneous mixtures can be provided with ceramicpowders up to about 44 micrometers (-325 mesh).

The SiC whiskers used in the present invention are single crystalscontaining beta and mixed alpha and beta phases of silicon carbide. Theaverage diameter of the whiskers is 0.6 micrometers and a length of10-80 micrometers with an average aspect ratio of 75. The whiskercontent in an average lot is 80-90% with the rest being formed ofsilicon carbide powders. Chemical analysis of the whiskers showed majorimpurities of oxygen: 0.1 wt. %; Ca, Mn, Mg, Fe, Al: 0.1-0.8 wt. %; Cr,K, Cu, Ni, Na: 100-1,000 ppm. These SiC whiskers are manufactured fromrice hulls and are commercially available as grade F-9 (SC-9) whiskersfrom ARCO Metals, Silag Operation, Greer, S.C. .[.or under the trademark"Tokamax" obtainable from Tokai Carbon Company, Tokyo, Japan, which hasa sales office in New York, N.Y..].. Because of the high purity of theSiC whiskers they are stable at temperatures up to 1800° C. in inertgases. Also, the thermal stability of the SiC whiskers in ceramicmatrices at processing temperatures up to about 1900° C. provides adistinct advantage over continuous polycrystalline SiC fibers thattypically degrade due to grain growth above about 1200° C. Reportedvalues for the mechanical properties of the relatively longer SiCwhiskers indicate that the average tensile strength is of 7 GPa (1 Mpsi)and an elastic modulii of 700 GPa (100 Mpsi) are obtained.

The concentration of the SiC whiskers in the composite is in the rangeof about 5-60 vol. % and preferably about 5-40 vol. % with about 20 vol.% providing the best results. With SiC whisker concentrations greaterthan about 40 vol. % it is difficult to hot press the composites todensities greater than 99% of the theoretical density of this ceramicmaterial material and with concentrations greater than about 60 vol. %SiC whiskers considerable whisker clumping occurs which detracts fromthe composite toughness. With less than about 5 vol. % whiskersinsufficient toughness is achieved due to the low concentration of theSiC whiskers in the matrix which will expose an insufficient number ofwhiskers in the crack plane to adequately absorb the cracking energy.

The mixing of the SiC whiskers with the ceramic powders can be providedby employing any suitable mixing techique which will provide ahomogeneous dispersion of the whiskers in the matrix powders withminimal agglomeration and whisker clumping. For example, suitablemixtures may be formed by using a Waring blender, a mixing medium and anultrasonic homogenizer, with the best mixing being achieved when usingthe fine (0.5 to 1.0 micrometer) powders. In a typical mixing operationa predetermined amount of SiC whiskers and ceramic matrix powders aremixed in hexane in the blender at a rotational speed of 19,000 rpm for 2minutes. This mixture was then further dispersed in suitable ultrasonichomogenizer until the container became warm and thewhisker-powder-hexane mixture appeared "milky" and uniformly mixed. Thismixture was then dried by evaporation with constant agitation underflowing air. Alternatively, distilled water used as the mixing mediumwith freeze-drying provided similar results. Also, dry mixing or mixingwith other volatile media may be satisfactorily utilized to obtain ahomogeneous mixture with minimal SiC whisker clustering oragglomeration. With any mixing techniques the mixing becomes moredifficult for providing a homogeneous dispersion of the whiskers as thewhisker content increases due to the clustering of the whiskers intobundles within the mixture.

Upon completing the mixing operation, the mixture was formed into asuitable article configuration and hot-pressed to a density of greaterthan 99% of the theoretical density of the material. Hot pressing wasfound to be necessary to provides composites with essentially the fulltheoretical density of the ceramic since green densities of less thanabout 50% theoretical density were obtained with conventional compactiontechniques utilizing pressure loadings up to 210 MPa. It is necessary toprovide composites with greater than about 99% of the theoreticaldensity of the ceramic matrix material to obtain the maximum toughnesswith the minimum presence of porosity and other flaws which detract fromthe toughness of the composite. High density is also required from thestandpoint of strength.

The hot pressing step may be achieved in a suitable induction orresistance heated furnace with punches or pressing components formed ofgraphite or any other suitable material which is capable of withstandingthe required pressures and temperatures without adversely reacting withthe composite constituents. For example, specimens, 38 mm in diameter by13 mm thick, 25 mm in diameter by 13 mm thick, and 13 mm in diameter by19 mm in length were hot pressed in a graphite resistance furnace withgraphite punches and dyes lined with "Grafoil" a trademarked product ofUnion Carbide Corporation, New York, N.Y. Rectangular specimens of about75 mm by 12 mm were made using round dyes with cheek pieces. Thepressing was achieved in a vacuum furnace of less than about 1.3 mPa attemperatures ranging from about 1600° to 1950° C. and at pressures inthe range of about 28 to 70 MPa for about 0.75 to 2.5 hours dependingupon the matrix material. During the hot pressing step a pressingpressure of about half of the predetermined total pressure was appliedto the composite until the composite reached the desired hot-pressingtemperature, then full pressure was applied. Densification of thecomposite may be monitored by a linear variable displacement transducerattached to the top ram of the press.

The above-described hot pressing operation and the Examples below aredirected to unidirectionally or uniaxially hot pressing the mixture forproviding composites in which the whiskers are preferentially alignedand randomly distributed in a plane or axis perpendicular to the hotpressing axis. However, it is expected that satisfactory compositeswhich exhibit increased toughness will be provided by employingisostatic hot-pressing techniques which provide for the omnidirectionalorientation of the SiC whiskers within the composite. This orientationof the whiskers in the composite is particularly desirable for thefabrication of complex shapes such as turbine blades which are exposedto cracking stresses from various angles. In such isostatic hot-pressingoperations, which may be readily achieved in a high temperatureinert-gas autoclave, the pressures and temperatures applied to themixture enclosed in a metal can such as tantalum to provide compositesgreater than about 99% theoretical density are expected to be in thesame range as the pressures, durations, and temperatures used in theuniaxial pressing operations.

The stability of the SiC whiskers in the ceramic matrices duringprocessing at temperatures up to about 1900° C. was found to be highlydesirable. The SiC whiskers were very stable at these elevatedtemperatures because they are single crystals and do not degrade attemperatures greater than about 1250° C. due to grain growth whichcommonly occurs in continuous polycrystalline SiC fibers. Also, sincethe whiskers had a relatively low oxygen content as set forth above theydid not react with the ceramic composites so as to effect decompositionof the whiskers. The attribute of the thermal stability of the SiCwhiskers at such elevated temperatures which were necessary forhot-pressing composites to densities greater than 99% theoreticaldensity is a significant factor in the successful development of the SiCwhisker reinforced composites with improved fracture toughness andstrength.

In order to provide a more facile understanding of the present inventionexamples are set forth below directed to the fabrication of SiCwhisker-composite with various ceramic materials. As mentioned above,the hot-pressing technique employed for fabricating these composites isthe use of a uniaxial applied pressure for providing composites withwhiskers predominantly oriented in a plane orthagonal or perpendicularto the hot-pressing axis.

EXAMPLE I

Forty grams of Al₂ O₃ powder (˜0.3 μm in size) was dry mixed with 8.2 gor 20 vol. % SiC whiskers. The mixture was uniaxially hot-pressed at atemperature of 1850° C. under a pressure loading of 41 MPa for 45minutes. The microstructure of the resulting composite consisted of SiCwhiskers in a fine-grain (about 10 micrometers) Al₂ O₃ matrix. The bulkdensity was over 99.7% of the theoretical density of which is 3.97mg/m³. The fracture toughness (K_(Ic)) for crack propagation along thehot-pressing axis at room temperature was 8 to 9 MPa.m^(1/2) and theroom temperature facture strength (σ_(f)) was 100 ksi.

Several pressings made with Al₂ O₃ powder as in Example I but withdifferent whisker concentrations, temperatures, pressures and ceramicconstituents are set forth in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Hot-pressing results of SiC-whisker Al.sub.2 O.sub.3 -matrix                  composites.sup.a                                                                            Hot Pressing                                                    Sample                                                                              SiC               Temper- Pres- Density                                 No.   whisker  Al.sub.2 O.sub.3                                                                       ature   sure  (Mg/ %                                  (SC)  vol. %   type     (°C.)                                                                          (MPa) m.sup.3)                                                                           T.D.                               ______________________________________                                         51   20       Linde-A.sup.b                                                                          1900    62    3.77 98.7                                61   20       "        1600    41    3.62 94.8                                68   20       "        1750    62    3.81 99.8                                76   20       "        1750    69    3.81 99.8                                79   20       "        1750    41    3.71 97.0                                83   20       "        1850    41    3.82 99.9                                89   20       "        1850    59    3.82 99.9                                90   10       "        1850    59    3.90 99.8                                91   30       "        1850    59    3.73 99.8                                92    0       "        1850    59    3.97 100                                123   20       "        1850    59    3.82 99.9                               124   20       "        1850    62    3.82 99.9                               134   20       "        1850    28    3.81 99.8                               135   20       "        1850    41    3.82 99.9                               136   20       "        1850    55    3.82 99.9                               137   10       "        1850    28    3.90 99.8                               138   10       "        1850    41    3.90 99.8                               139   10       "        1850    55    3.90 99.8                               140   30       "        1850    28    3.67 98.2                               141   30       "        1850    41    3.73 99.8                               142   30       CR-10.sup.c                                                                            1850    55    3.73 99.8                               146   20       "        1850    41    3.82 99.9                               154   20       "        1850    41    3.82 99.9                               156    5       "        1850    41    3.94 99.9                               157   10       "        1850    41    3.90 99.9                               158   20       "        1850    41    3.82 99.9                               159   30       "        1850    41    3.74 99.9                               162   40       "        1850    41    3.66 99.9                               163   50       "        1850    41    3.58 99.7                               164   60       "        1850    41    3.43 97.5                               ______________________________________                                         .sup.a Hotpressing time, 45 minutes                                           .sup.b Union Carbide Corporation. Indianapolis, Indiana (avg. particle        size 0.3 m. T.D. 3.97 Mg/m.sup.3)                                             .sup.c Backowowski International Corporation. Charlotte, North Carolina       (avg. particle size 0.2 m. T.D. 3.97 Mg/m.sup.3)                         

EXAMPLE II

Carborundum B₄ C powder (-44 micrometer, 12.1 g) was mixed with siliconcarbide whiskers (3.9 g or 20 vol. %) and hexane (400 mL) in a Waringblender and then slurry dried. The mixture was uniaxially hot-pressed ata temperature of 1900° C. under a pressure loading of 62 MPa for onehour. The density of the hot-pressed composite was greater than 99.7% ofthe theoretical density which is 2.66 Mg/m³. The K_(Ic) is 5.3MPa.m^(1/2) and the σ_(f) is 46 ksi at room temperature.

EXAMPLE III

Mullite powder (3 Al₂ O₃.2SiO₂) of a size less than 44 μm was mixed with2.5 g or 20 vol. % SiC whiskers in hexane using a Waring blender andthen slurry dried. The composition or mixture was uniaxially hot-pressedat a temperature of 1600° C. under pressure loading of 70 MPa for onehour. The density of the hot-pressed composite was greater than 99.8% ofthe theoretical density of mullite which is 3.08 Mg/m³. The K_(Ic) was4.6 MPa.m^(1/2) and the σ_(f) was 63 ksi at ambient temperature.

The fracture toughness values of the 20 vol. % SiC whisker Al₂ O₃ - andmillite-matrix composites were substantially greater than those of thesingle-phase monolithic ceramics as shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        K.sub.Ic and σ.sub.f of 20 vol. % SiC-whisker Al.sub.2 O.sub.3 --,      mullite-, and B.sub.4 C-matrix composites                                                          K.sub.Ic   σ.sub.f                                 Material             (MPa · m.sup.1/2)                                                               (MPa)                                         ______________________________________                                        20 vol. % SiC whisker CR-10 Al.sub.2 O.sub.3                                                       9.0        805 ± 94                                   20 vol. % SiC whisker Linde-A Al.sub.2 O.sub.3                                                     8.6        600 ± 10                                   Al.sub.2 O.sub.3     4.6                                                      20 vol. % SiC whisker mullite                                                                      4.6        438 ± 5                                    composite                                                                     Mullite              2.2                                                      ______________________________________                                    

The toughness as expected was a function of the orientation of the crackplane and direction. With these values in Table 2 being obtained fromspecimens in which the crack propagation direction was normal to thehot-pressing axis while the crack plane was parallel to the hot-pressingaxis. However, even when the crack propagated along the plane of thewhiskers (parallel to the hot-pressing axis) average K_(Ic) value was5.5 MPa.m^(1/2) for the 20 vol. % SiC whisker Al₂ O₃ composites versusapproximately 4.5 MPa.m^(1/2) for pure Al₂ O₃. Observation of fracturesurfaces indicated significant whisker pull-out in the two types ofcomposites listed in Table 2.

It will be seen that the present invention provides for the fabricationof highly dense SiC whisker-ceramic composites which provide asignificant increase in fracture toughness over the monolithic ceramicmaterial. It is expected that these composites will be of a significantvalue in the form of production components which are subjected to hightemperatures and pressures. Also, the fracture toughening process(whisker pull-out) in the composites is activated during slow crackgrowth (fatigue) and substantially increase the resistance to slow crackgrowth in the composite. Furthermore, the increased slow crack growthresistance associated with the increased fracture toughness wouldsubstantially increase the lifetime of these ceramics when they aresubjected to various conditions under applied stress such as in heatengines and the like. The resulting increase in the fracture toughness(K_(Ic)) provided in the composites of the present invention allows forhigh-fracture strength components to be fabricated which have largerflaw or defect sizes than allowable in conventional ceramic composites.This flow size allowance results from a dependence of flaw size (c) fora given desired fracture strength (K_(Ic)) which is provided by theformula σ_(Ic) Y K_(Ic) c⁻ 1/2 where (Y) is a geometry factor. Thus, forfracture strengths of 700 MPa the allowable flaw size can be increasedby a factor of 5, i.e., from about 50 micrometers to 250 micrometers byincreasing the fracture toughness from 4 MPa.m^(1/2) to about 9MPa.m^(1/2).

I claim:
 1. A ceramic composite characterized by increased toughness andresistance to fracture, comprising a composite defined by a matrix ofceramic material, .Iadd.wherein said matrix is selected from the groupconsisting of A₂ O₃, mullite and B₄ C .Iaddend.having homogeneouslydispersed therein about 5 to 60 vol. % of silicon carbide whiskers, withsaid silicon carbide whiskers having a microcrystalline structure and.[.in.]. a size range of about 0.6 μm in diameter and a length of 10 to80 μm .Iadd.and which are characterized by crack bridging pull-out fromsaid matrix when said matrix is subjected to crack-forming stresses,.Iaddend.said composite being of a density greater than about 99% of thetheoretical density.
 2. The ceramic composite claimed in claim 1,wherein the silicon carbide whiskers are .[.essentially unidirectionallyoriented.]. .Iadd.preferentially aligned and randomly distributed inplanes .Iaddend.in the matrix.
 3. The ceramic composite claimed in claim2, wherein the matrix is Al₂ O₃, the concentration of silicon carbidewhiskers in the matrix is about 20 vol. %, wherein fracture toughness ofthe composite is in the range of about 8 to 9 MPa.m^(1/2) at roomtemperature, and wherein fracture strength of the composite is in therange of about 590 to 899 MPa at room temperature.
 4. The ceramiccomposite claimed in claim 2 wherein the matrix is 3Al₂ O₃.2SiO₂, theconcentration of silicon carbide whiskers in the composite is about 20vol. % wherein fracture toughness of the composition is 4.6 MPa.m^(1/2)at room temperature, and wherein fracture strength of the composite isabout 438 MPa at room temperature.
 5. The ceramic composite claimed inclaim 1, wherein the silicon carbide whiskers are essentiallyomnidirectionally oriented in the matrix. .[.6. The ceramic compositeclaimed in claim 1, wherein the ceramic matrix material is selected fromthe group consisting of Al₂ O₃, 3Al₂ O₃.2SiO₂, B₄ C..].
 7. A method forpreparing a ceramic composite having increased toughness and resistanceto fracture, comprising the steps of forming a homogeneous mixture of.[.particulate ceramic material with.]. about 5 to 60 vol. % of siliconcarbide whiskers having a monocrystalline structure and .[.in.]. a sizerange of about 0.6 μm in diameter and a length of 10 to 80 μm .Iadd.withparticulate matrix wherein said matrix is selected from the groupconsisting of Al₂ O₃, mullite and B₄ C, .Iaddend.and hot pressing themixture at a pressure in the range of about 28 to 70 MPa and temperaturein the range of about 1600° to 1900° C. for a duration of 0.75 to 2.5hours to provide a composite with a density greater than about 99% ofthe theoretical density of the ceramic material .Iadd.and in whichcomposite the silicon carbide whiskers are characterized by crackbridging pull-out from the ceramic material when said composite issubjected to crack-forming stresses..Iaddend.
 8. A method for preparinga ceramic composite as claimed in claim 7, wherein the ceramic materialis of a particle size less than about 44 m. .[.9. A method for preparinga cermamic composite as claimed in claim 8, wherein the ceramic materialis selected from the group consisting of Al₂ O₃, 3Al₂ O₃.2SiO₂, and B₄C..].
 10. A method for preparing a ceramic composite as claimed in claim.[.9.]. .Iadd.7.Iaddend., wherein the silicon carbide whiskers arepreferentially aligned along an axis of the composite byunidirectionally hot pressing the mixture along an axis peripendicularto the first-mentioned axis.
 11. The method for preparing a ceramiccomposite as claimed in claim .[.9.]. .Iadd.7.Iaddend., wherein thesilicon carbide whiskers are omnidirectionally aligned with thecomposite by isostatically hot pressing the mixture.